1. Field of the Invention
The present invention relates to (ethylene, vinyl acetal) copolymers and to lithographic printing plate precursors including such copolymers.
2. Description of the Related Art
Lithographic printing typically involves the use of a so-called printing master such as a printing plate which is mounted on a cylinder of a rotary printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-adhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Lithographic printing masters are generally obtained by the image-wise exposure and processing of an imaging material called plate precursor. The coating of the precursor is exposed image-wise to heat or light, typically by means of a digitally modulated exposure device such as a laser, which triggers a (physico-)chemical process, such as ablation, polymerization, insolubilization by cross-linking of a polymer or by particle coagulation of a thermoplastic polymer latex, solubilization by the destruction of intermolecular interactions or by increasing the penetrability of a development barrier layer. Although some plate precursors are capable of producing a lithographic image immediately after exposure, the most popular plate precursors require wet processing since the exposure produces a difference of solubility or of rate of dissolution in a developer between the exposed and the non-exposed areas of the coating. In positive working plates, the exposed areas of the coating dissolve in the developer while the non-exposed areas remain resistant to the developer. In negative working plates, the non-exposed areas of the coating dissolve in the developer while the exposed areas remain resistant to the developer. Most plates contain a hydrophobic coating on a hydrophilic support, so that the areas which remain resistant to the developer define the ink-accepting, printing areas of the plate while the hydrophilic support is revealed by the dissolution of the coating in the developer at the non-printing areas.
Many lithographic printing plates contain polymeric binders such as phenolic resins which can be baked in order to increase the run length on the press. Over the last few years, printing plate materials which provide a high run length without baking have become more popular because the post-bake oven can be eliminated leading to reduced energy consumption and less floor space. The trend towards higher printing speeds on web presses and the use of recycled paper require plate coatings that are characterised by a high abrasion resistance. Unbaked phenolic resins such as novolac, resol or poly(vinyl phenol) have a poor abrasion resistance and cannot provide a high run length in such conditions.
In the prior art, the run length of lithographic printing plates based on phenolic resins has been improved by chemical modification of such binders. Examples thereof are described in for example WO 99/01795, EP 934 822, EP 1 072 432, U.S. Pat. No. 3,929,488, EP 2 102 443, EP 2 102 444, EP 2 102 445 and EP 2 102 446. Phenolic resins have also been mixed with or replaced by other polymers such as poly(vinyl acetal) resins in order to improve the abrasion resistance of the coating. Suitable poly(vinyl acetal) resins are described in U.S. Pat. No. 5,262,270; U.S. Pat. No. 5,169,897; U.S. Pat. No. 5,534,381; U.S. Pat. No. 6,458,511; U.S. Pat. No. 6,541,181; U.S. Pat. No. 6,087,066; U.S. Pat. No. 6,270,938; WO 2001/9682; EP 1 162 209; U.S. Pat. No. 6,596,460; U.S. Pat. No. 6,458,503; U.S. Pat. No. 6,783,913; U.S. Pat. No. 6,818,378; U.S. Pat. No. 6,596,456; WO 2002/73315; WO 2002/96961; WO 2003/79113; WO 2004/20484; WO 2004/81662; EP 1 627 732; WO 2007/17162; WO 2008/103258; U.S. Pat. No. 6,255,033; WO 2009/5582; WO 2009/85093; WO 2001/09682; US 2009/4599; WO 2009/99518; US 2006/130689; US 2003/166750; U.S. Pat. No. 5,330,877; US 2005/3296; WO 2007/3030; US 2009/0291387; US 2010/47723 and US 2011/0059399.
Poly(vinyl acetal) resins are prepared in the art by acetalisation of poly(vinyl alcohol) with aldehydes. Poly(vinyl acetals) used for lithographic printing plate coatings typically comprise both a hydrophobic acetal moiety, which provides the ink-acceptance, and an hydroxyl substituted aromatic acetal moiety, which produces the solubility differentiation in an alkaline developer upon exposure.
Such poly(vinyl acetal) resins are typically prepared by the acetalisation of poly(vinyl alcohol) with a mixture of aldehydes, e.g. an aliphatic aldehyde such as butyraldehyde mixed with a phenolic aldehyde such as hydroxybenzaldehyde. The physical and chemical properties of such poly(vinyl acetal) resins are highly dependent on the degree of acetalysation, the ratio of the aliphatic and the aromatic acetal moieties, the stereochemistry and the random or block nature of the acetal resin. Small shifts in process conditions during the preparation of the known acetal resins may produce significant differences in the structure of the obtained resin and thus in significant differences of their properties. For example, incomplete dissolution of the poly(vinyl alcohol) reagent may lead to an irreproducible degree of conversion, i.e. a lack of control of the composition of the final product. Also the competition and the transacetylisation which often occurs between the mixed aldehyde reagents is difficult to control so that the right balance between the hydrophobicity of the resin and its solubility in an alkaline developer cannot always be obtained.
In the unpublished patent application PCT/EP2013/075366 filed on Jan. 1 2013, (ethylene, vinyl acetal) copolymers and their use in lithographic printing plate precursors are disclosed.
With the so-called thermal digital plates, an infrared laser is used to image infrared radiation sensitive precursors. These infrared radiation sensitive precursors have as a common ingredient a compound that triggers the imaging mechanism by absorbing and converting the infrared radiation, which is used to image the precursors, into heat. Such compounds are often dyes, commonly referred to as IR-dyes. It has been observed that such IR-dyes, present as major ingredients in the coating of the precursors, can also have a negative impact on these precursors. For example, it has been observed that IR-dyes may be inhomogeneously distributed or even form aggregates in the coating of a printing plate due to their poor solubility in common coating solvents. Such aggregates may tend to form so-called hot spots in the coating leading to unwanted partial ablation. The generation of ablation debris may contaminate the electronics and optics of exposure devices. In addition, IR-dyes may form crystals in the coating which may locally influence the solubilisation behaviour of the coating. As a result, a poor differentiation between image and non-image areas may be obtained leading to a poor printing quality. The occurrence of these crystallization defects may become more pronounced when the printing plate precursor is stored before exposure and development, i.e. the storage stability of the precursor is poor.
EP 1 297 950 discloses a heat-sensitive lithographic printing plate precursor comprising a polymer which is soluble in an aqueous alkaline solution and which comprises at least one chromophoric moiety having a light absorption maximum in the wavelength range between 400 and 780 nm.
U.S. Pat. No. 6,124,425 discloses a reactive infrared absorbing polymer having a molecular weight of more than 5000 which comprises a near infrared absorbing segment, a processing segment and a thermally reactive segment.
WO 2001/94123 discloses a printing form precursor which includes on a substrate a thermally imageable coating including a polymer comprising a pendant infrared absorbing agent and a cover layer. Such precursors avoid migration of the dyes and errors in manufacturing leading to inconsistent batches.